Now, depcomp adds
`-Wp,-MDupdate,.deps/libIlmImf_la-ImfAttribute.TPlo' to
the end of the command line before handing it to
libtool. libtool, then, mistakes that option for a
source file. This gets important when compiler_c_o==no
(compiler can't use -c and -o at the same time), and
the object file has a different basename than the
source ($CC -o newname.o -c oldname.c).